Did you know that plants emit airborne distress signals when they are getting eaten? When damaged, plants release volatile organic compounds (VOCs) and according to a new study, these compounds can serve two functions, one to attract enemies that might attack the herbivorous insects eating the plant, and two, to ward off the herbivorous insects, which avoid the herbivore-induced VOCs.

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A team of researchers has found that the odor released by maize plants under attack by insects attract not only parasitic wasps, which prey on herbivorous insects, but it also attracts caterpillars of the Egyptian cotton leafworm moth Spodoptera littoralis, a species that feeds on maize leaves.

"Adult moths and butterflies avoid food plants that are under attack by conspecifics. This seems adaptive, because it reduces both competition and the risk of predation by parasitoids. But we found that S. littoralis caterpillars are actually attracted to the odor of damaged maize plants, even when this odor is mimicked in the laboratory with a mix of synthetic compounds," said Professor Ted Turlings, an author of the study and head of the Laboratory for Fundamental and Applied Research in Chemical Ecology Institute of Biology at the University of Neuchâtel, Switzerland.

Researchers let the caterpillars chose among several odors by placing them in an olfactometer, a device with four distinct tube that had an airflow carrying a different odor. Results show that caterpillars were more than twice as likely to crawl towards the odor from maize plants under attack than towards undamaged plants, especially if the damage was recent and the caterpillars had already fed on maize.

"When S. littoralis caterpillars drop from a plant they are highly vulnerable to predators and pathogens in the soil, as well as to starvation. The advantage seems to be that fallen caterpillars can quickly rediscover the plant on which they fed. The caterpillars feed less and move more when exposed to high concentrations of the volatiles. By moving away from freshly damaged sites, they can minimize risk of predation and avoid competition," explained Prof. Turling.

Researchers propose that hungry S. littoralis caterpillars move towards VOCs released by damaged maize plants because even though competition may be more intense, the caterpillars are guaranteed a suitable plant. Adult moths, on the other hand, are much more mobile and take little risk exploring the environment to discover the best food source - so they avoid maize that is already under attack.